Input: atmel_mxt_ts - return errors from i2c layer

The i2c layer can report a variety of errors, including -ENXIO for an i2c
NAK.  Instead of treating them all as -EIO, pass the actual i2c layer
error up to the caller.

However, still report as -EIO the unlikely case that a transaction was
partially completed, and no error message was returned from i2c_*().

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>